A digital image sensor is a semiconductor device comprising an array of pixels, each of which comprises a photosensitive element (e.g. a photodiode) for converting incident light into a photocurrent. The generated photocurrent is gathered for an exposure time before being converted to a voltage, digitized and read out. Different architectures of digital image sensor are defined by the processes used in their manufacture. Two of the most common types of digital image sensor are the charge coupled device (CCD) and the Complementary Metal Oxide Semiconductor (CMOS) image sensor. CMOS image sensors are particularly applicable for applications which require low power operation at low cost.
A digital image sensor is formed as an integrated circuit (IC), and an imaging device typically comprises the digital image sensor IC together with optical components (e.g. a lens) and other device components.
A digital image sensor comprises a number of well known volatile memory components, i.e. memory components that only maintain the stored data for as long as a power supply is maintained. In particular, for a CMOS sensor, each pixel comprises capacitors which store charge for subsequent readout. The readout operation is controlled with vertical and/or horizontal shift registers. Line memories are required for functions such as analog to digital conversion (ADC), digital filters, and defect correction algorithms. A frame store can be provided which stores image data for correlated double sampling or for schemes which combine images to extend dynamic range. This is not an exhaustive list of the types of volatile memory, or of the uses to which each is put.
For the permanent storage of data, non-volatile memory is used. Non-volatile memories are typically used in read-only memory (ROM), flash memory, magnetic computer storage devices such as hard disks, floppy disk drives, or in optical disc drives.
Most non-volatile memory cells rely on a trapped charge stored on a floating gate in a field effect transistor (FET), and so the contents of the memory are erased if exposed to incident light. Because of this, non-volatile memory is not generally thought of as being suitable for use with an image sensor. If any extra memory storage is required for a given image sensing device, the non-volatile memory needs to be stored on a separate IC, remote from the image sensor IC. The use of a multiple chipset package increases the size and manufacturing costs of an image sensing device. U.S. Pat. No. 6,879,340 to Chevallier discloses a CMOS image sensor that incorporates a non-volatile memory, which is used as a frame store. In order to enable the use of a non-volatile memory, the disclosure of Chevallier provides a layer of opaque protective material which covers the non-volatile memory. This represents an extra processing step during sensor manufacture, and an increase in manufacturing complexity such as this increases the time of manufacture and decreases device yield and so from a cost and profitability perspective the avoidance of this step is desirable.
As described above, volatile memory is commonly used in image sensors for the performance of the sensor's image processing tasks. U.S. Pat. No. 6,879,340 to Chevallier discloses an image sensor IC which comprises a non-volatile memory which is also used for the sensor's image processing tasks. There is a need for a non-volatile memory provided on an image sensor IC which can perform other tasks.